Radio transmissions, especially frequency-modulated (FM) radio transmissions in the ultra-shortwave region, are already being transmitted with an RDS signal by almost all transmitters. The RDS signal causes the program name (e.g., SWF3 as an abbreviation for Sudwestfunk, Program 3) to appear in clear text on the display of a radio receiver. This is transmitted in a maximum of 8 ASCII characters, but is the same for all transmitter locations of the given program, i.e., the transmitter location (antenna) is not transmitted.
In addition, it is well known that a transmitter's field strength decreases hyperbolically, i.e., nonlinearly, with increasing distance from the transmitter.
For vehicles, especially motor vehicles, location position determination is becoming increasingly important for the purpose of making a direct determination of the location, when, for example, an accident site needs to be reported to a rescue station, or, in international freight traffic, the present position of a truck needs to be reported to the truck fleet management control center. On the other hand, indirect position determination is necessary in certain situations, e.g., when from among a large number of traffic messages, it is necessary to select only those messages that are relevant to the present position of the vehicle. ARI radio traffic messages are presently still being given in spoken form, but in the future they will be permanently transmitted as inaudible digital "radio data system/traffic message channel" signals (RDS/TMC signals). In this regard, it can be expected that the number of radio traffic messages will continue to increase, so that a selection of incoming radio traffic messages seems absolutely necessary, and this selection can be made, e.g., indirectly by a position determination.
One well-known method of position determination is the Global Positioning System (GPS). This system requires special receivers and about 24 satellites in orbit, which is a very expensive method. Another method makes position determinations by measuring the travel time of radio signals with
a) synchronized radio transmitters, which requires the conversion of many transmitters,
b) nonsynchronous radio transmitters, which requires precise distance determination.
Methods a) and b) both require high-quality measuring technology for measuring the travel time. Another well-known method is the dead-reckoning navigational system, but this is very expensive.
DE-OS38 37 633 describes a location and navigation system for mobile radio stations, which frees the driver from searching for the right map section. Location information corresponding to the approximate position of the vehicle is developed from information exchanged between mobile radio stations and stationary radio stations, and then this location information is used to automatically select the relevant map section.